When a wiring process or the like is performed on a substrate such as a semiconductor wafer, it is required to perform a fine process on the substrate and, thus, a substrate processing method using a plasma has been widely employed.
This substrate processing method uses a technique for generating a plasma in a pattern of a pulse wave in order to meet the requirement for accuracy of a processed shape or the like in a RIE (Reactive Ion Etching) process. By generating a plasma in the pattern of a pulse wave, it is possible to properly control dissociation state of a plasma generating gas. Accordingly, over-etching can be suppressed, and a fine process can be performed.
Recently, there is suggested a synchronous pulse control that combines a technique for generating a plasma in the pattern of a pulse wave by applying a high frequency power for plasma generation (hereinafter, referred to as “source RF”) in the pattern of a pulse wave and a technique for controlling attraction of positive ions in the plasma in the pattern of a pulse wave by applying a high frequency power for biasing (hereinafter, referred to as “bias RF”) in the pattern of a pulse wave (see, e.g., Patent Document 1).
Patent Document 1: Japanese Patent Laid-open Publication No. 2000-311890
However, in the reactive ion etching process, when attracting positive ions in the plasma to the semiconductor wafer, positive ions 72 may remain at a bottom of a hole 71 formed in a target film 70 as shown in FIG. 7. The remaining positive ions 72 may electrically prevent other positive ions 73 from reaching the bottom of the hole 71, so that the path of the other positive ions 73 in the hole 71 may be changed. Accordingly, the hole 71 is deformed, which results in the decrease in the etching efficiency by the positive ions.
Meanwhile, in the synchronous pulse control type plasma etching, after the application of the source RF is stopped (OFF), deactivated electrons that have lost energy in the plasma may adhere to electrically neutral molecules, atoms, or radicals, so that negative ions are generated. Therefore, it can be considered to increase the etching efficiency by using the negative ions in order to electrically neutralize the positive ions remaining at the bottom of the hole 71.